1. Field of the Invention
The present invention relates to a surface fastener obtained by warp knitting and more particularly to a knitted surface fastener that secures flexibility characteristic to knitted fabric without necessity of back coating.
2. Description of the Related Art
In a surface fastener made of fiber, engaging element forming yarns are woven or knitted in so as to form a plurality of loops on a surface of a base fabric at a same time when the base fabric is woven or knitted. Each of the loops serves as a female engaging element as it is or it forms a hook-like or mushroom-shaped male engaging element by cutting a part thereof. In order to prevent the engaging element from loosing out of the base fabric in the surface fastener obtained by weaving or knitting, a surface opposite to the surface having the engaging elements is coated with resin or so-called back coating is performed so as to bond the base fabric with the engaging elements as disclosed in for example, Japanese Utility Model Publication No. 2563369.
This back coating can prevent the engaging elements from loosing out of the base fabric. However, it is usually necessary to apply resin over an entire surface of the base fabric in order to prevent the engaging elements from loosing out. As a result, the entire base fabric hardens so that gaps between stitches or weave patterns are covered with resin except the aforementioned Japanese Utility Model Publication No. 2563369 disclosing a net-like base fabric, and as a consequence, ventilation becomes poor. Thus, a surface fastener which prevents the engaging elements from loosing out without necessity of back coating and at the same time, secures flexibility particular to knitted or woven fabric has been demanded strongly.
For example according to Japanese Patent Application Laid-Open No. 63-200705, knitting yarn strides over wales of first to third chain knitting yarns so as to form a loop on a same course. More specifically, after yarn for forming an engaging element is knitted in a zigzag shape into three courses adjoining each other on a second wale, it moves to an adjoining first wale while forming a loop. Then, it is knitted into the three courses in the zigzag shape and returns to the original second wale while making a loop and there, the three courses are knitted in the zigzag shape and the knitting stage moves to the adjoining first wale and this pattern is repeated.
While the yarn for forming the engaging elements is knitted into the three courses on the first and second wales in the zigzag shape, a second yarn for forming engaging elements is operated in a same way between the second and third wales so as to form a loop. Further, according to the Japanese Utility Model Publication No. 2563369, a heat shrinkage ratio of a warp yarn of the base fabric and loop yarn is set higher than that of a weft yarn in order to prevent the engaging elements from loosing out, and thereby the loop yarn is fixed by heat shrinkage. By adopting such a structure, the loop is fixed to the base fabric without back coating. However, plural loop yarns are not knitted into the same course on the same wale formed with the chain knitting yarns but a single loop yarn is knitted into each course and therefore, the engaging elements are likely to loose out.
Likewise, Japanese Patent Application No. 2003-274704 has proposed a warp knitted surface fastener made of fibers equipped with a knitting construction in which a loop is fixed to a base fabric securely without necessity of back coating or use of any particular heat shrinkage yarn.
According to the invention of the Japanese Patent Application No. 2003-274704, a loop is formed across two or more wales with a pile knitting yarn for forming a pile loop and after that, the pile knitting yarn is entangled with each stitch of adjoining three or more courses on the wales continuously so as to form a stitch. Among the stitches of the pile knitting yarn formed when the pile knitting yarn is entangled with each of the adjoining three or more courses, at least one or more stitches located in a middle are formed with a closed loop. Thus, if the stitch at a loop end formed across an adjoining wale is of an open loop, even if that loop is pulled strongly, the stitches formed with the aforementioned closed loop in the middle is tightened from both directions, the stitch is fixed tightly so that it is not pulled out. As a result, the base portion of the pile loop is firmly fixed not only to a base fabric having a high density knitting structure but also to a net-like base fabric, and even if no resin coating is performed or adjoining yarns are not fused together, prevention of the loosing out of the engaging element is carried out securely.
Although, according to the Japanese Patent Application No. 2003-274704, the loop yarn is securely prevented from loosing out because the loop forming yarn is entangled with three or more courses adjoining a chain knitting yarn constituting the wale and a stitch located in the middle of those stitches is entangled with a stitch of chain knitting yarn with a closed loop, the stitch formed with the closed loop not only increases a thickness of the base fabric due to its stitch structure but also increases its hardness, thereby likely damaging flexibility required for knitted fabric.